Uninterruptible power supplies are in common use today particularly for supplying large computer systems where loss of line power can result in the interruption of programs and loss of valuable data. Such uninterruptible power supplies may also provide a signal conditioning function to ensure that transient spikes, low voltage conditions, or distorted power waveforms on the AC power system do not disturb the operation of the computer to which the uninterruptible power supply (UPS) is connected. Typically, the UPS includes a battery which is interfaced through an inverter to the AC output power line. One type of UPS operates in such a way that when a fault occurs in the input AC power, the inverter is controlled to provide power from the battery to the AC output line which has the same frequency and substantially the same waveform as the normal input AC power. Preferably, the switching at the time of fault is accomplished as smoothly as possible so that substantial transient spikes or dips in the waveform supplied to the AC output lines does not occur. The coupling of the inverter to the AC output may be through a ferroresonant transformer as illustrated in U.S. Pat. No. 4,692,854, to Richard V. Baxter et al. entitled METHOD AND APARATUS FOR MODULATING INVERTER PULSE WIDTH, the disclosure of which is incorporated herein by reference.
A major factor in the ability of a UPS to switch smoothly from failing line power to battery power is sufficiently rapid detection of the line power fault. Preferably, power can be switched to the battery backup within a small fraction of a cycle while nonetheless maintaining a system which is relatively insensitive to minor transient conditions and disturbances which are not indicative of a significant fault. An overly sensitive switching scheme would result in "false positives" such that the UPS would be switched in at a time when it is not necessary.
One approach which has been used for line fault detection is to provide a table of reference values for each cycle of the power waveform (e.g., the 60 Hz power waveform used in the United States). The voltage on the AC power line should conform substantially to a sine wave at the line frequency whose phase does not change. Each cycle of the waveform on the AC power line can then be sampled at specific points in time during the cycle and the values of the samples can be compared to the nominal table values, with a default being indicated when the difference at each of the sample times exceeds a predetermined tolerance limit.
Certain forms of waveform distortion are commonly seen which do not necessarily indicate a power line failure so that the UPS should not switch when such distortions occur. Two examples of such waveform distortion are shown in FIG. 1 which are commonly seen by a UPS. An idealized form of third harmonic distortion is shown by the waveform labeled 11 in FIG. 1. Quite often, this sort of distortion is caused by magnetic saturation of transformers and is usually more pronounced at the trailing side of the waveform. A flat-topped waveform 12 is also shown in FIG. 1 which can be caused by a non-linear load such as a switched-mode power supply. To avoid nuisance switching of the UPS where such distorted line voltage sources are present, special reference tables can be programmed into the controls for the UPS units so that they will not switch unnecessarily when such a waveform is present. In effect, the comparison between the input AC waveform and the reference has been "desensitized" so that a greater distortion of the waveform must occur before a fault will be found than ordinarily would be preferred, since the system will now be less sensitive to actual power line failure conditions which may mimic the forms of distortion shown in FIG. 1. For example, the waveform 13 shown in FIG. 2 shows the type of line failure which can occur in a network connected to rotating machinery or ferroresonant transformers. In this case, the line voltage rings-down at a rate which is dependent on the load. With a desensitized reference table, the voltage on the power line would need to decay to a level substantially lower than desirable before detection occurs. An undesirably large disturbance in the output voltage can result.
Another factor affecting detection of power line failure is the sensitivity of detection at various phase angles of each half-cycle of the input waveform. It has been found that in typical plots of cycle to cycle deviation of AC power, deviations generally do not occur during the peak of the waveform but rather are more prevalent toward the zero-crossings. In particular, with a ferroresonant transformer as the source, a small amount of phase perturbation can accentuate deviation around the zero-crossings. Thus, a detection method which would be based on percentage change of the waveform is generally not an effective way of fault detection. Generally, it is better to have an absolute difference of equal value for all samples as the criterion or, for severe cases, larger (programmable) tolerances for the differences at the zero crossings.
It is further desirable to minimize unnecessary switching of the UPS due to power line surges so that switching does not occur unless the surge represents a persistent change in the power system voltage.